This passage was revealed just after the battle of Uhud, where the Muslims lost. The Quraysh came from Makkah to Madinah to fight the Muslims, as a result of them losing previously in the battle of Badr.
Furthermore, scholars state the verse was revealed after the battle of Uhud (lesser Badr or third Badr), when the Quraysh became victories. They went back to Makkah and set another date that they would be coming back to fight the Muslims.
“So fight, [O Muhammad], in the cause of Allah ; you are not held responsible except for yourself. And encourage the believers [to join you] that perhaps Allah will restrain the [military] might of those who disbelieve. And Allah is greater in might and stronger in [exemplary] punishment.” – Quran 4:84
The verse highlights that some of the Muslims who were wounded at the battle of Uhud, didn’t want to participate in this. But the Prophet was encouraged to bring his people, that way they might put fear in Quraysh’s military, so that the enemy may abandon fighting the Muslims.
This verse clearly shows that the war was started by the Quraysh, not the Muslims. The clear words used here where the Prophet’s was encouraged to bring his people so that they may put fear in Quraysh’s military, so that they may abandon fighting the Muslims, shows that the Quraysh wanted a war, not the Muslims.
“100 Lit., “those from among them who elicit [the truth]”, i.e., the special organs of the state entrusted with gathering and evaluating political and military intelligence.
101 Although primarily addressed to the Prophet, the “thou” in this sentence relates to every believer. The above exhortation is to be understood in the context of a war already in progress, and not as an incitement to war.” 
Maarif-ul-Qur’an Tafhim – Mufti Mohammad Shafi Usman:
“THE BACKGROUND OF REVELATION
When the battle of Uhud came to pass in the month of shawwal, the Holy Prophet (p), in keeping with the promised date set by the disbelievers, started preparations to confront them at Badr in the month of Dhi-Qa’dah (historians identify which as the minor Badr). At that time, some people betrayed a little hesitation in going along because of having been recently wounded, while some others did so because of rumours. Thereupon, Allah Almighty revealed this verse where guidance has been given to the Holy Prophet (p) that he should ignore the infirm and the scared among Muslims and show no hesitation in marching on to Jihad even if it has to be all alone, for Allah is his Helper. With his guidance on hand, he went ahead to Badr Minor with seventy companions as promised to Abu Sufyan after the battle of Uhud. There it is so happened that Allah Almighty put awe and fear in the hearts of Abu Sufyan and his disbelieving accomplices from the tribe of Quraysh. Not one of them showed up for the fight as a result of which they turned out to be false in their solemn promise. Thus, as the text says, Allah Almighty prevented the mischief of the disbelievers and the Holy Prophet (p) returned back with his companions safely (Qurtubi & Mazhari).” 
Tafhim al-Qur’an – Sayyid Abul Ala Maududi:
“113 Different people behave differently and with different results. Some urge on others to exert themselves for the cause of Allah and to exalt His Word and they get its reward. There are others who spread misunderstandings among the people about Allah’s cause, discourage the Muslims and try to dissuade them from exalting the Word of Allah and thus incur punishment.
114 The Muslims were specially exhorted to be very civil and polite to the non-Muslims because at that time their relations were strained on account of the conflict between them. In that state of tension, they were forewarned to be on their guard against incivility and impoliteness. They were, therefore, taught to be equally civil and polite to them when they greeted them respectfully. Nay, they should be even more civil and polite than their opponents.” 
Malik Ghulam Farid:
“641. The command to fight does not relate to the Holy Prophet alone. If that had been the case, the second clause in the verse would have read as Illa Nafsuka, i.e., none is made responsible except thyself, and not as Illa Nafsuka, i.e., thou art not made responsible except for thyself, as in the verse. What the verse means is that every Muslim, not excluding the Holy Prophet, was individually answerable to God. But the duty of the Holy Prophet was twofold, (1) to fight himself, and (2) to urge his followers to fight, though he was not responsible for them.” 
Maulana Muhammad Ali:
“84a. These words show how great was the confidence of the Prophet in the ultimate triumph of the noble cause with which he was entrusted. The primary duty to defend Islam lay on him alone, as against all the hosts of Arabia. This shows that he never put any trust in the prowess of his followers, and his confidence was based solely on Divine help. With no material resources, he was sure not only that he would be able to restrain the fighting of his powerful opponents, but also that they would meet with condign punishment at his hands.” 
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 The Message of The Quran translated and explained by Muhammad Asad, page 184 – 185
 Maarif-ul-Qur’an Tafhim by Mufti Mohammad Shafi Usman, volume 2, page 522
 Sayyid Abul Ala Maududi – Tafhim al-Qur’an – The Meaning of the Qur’an
 The Holy Qur’an – Arabic Text With English Translation & Short Commentary By Malik Ghulam Farid,
 The Holy Quran Arabic Text with English Translation, Commentary and comprehensive Introduction [Year 2002 Edition] by Maulana Muhammad Ali, page 220